1. Field of the Invention
This invention relates to a rear projection screen and a process for producing the same. More particularly, this invention relates to a rear projection screen comprising a light scattering member composed of a melted mixture containing at least two high polymers having different refractive indexes, and a process for producing the same.
2. Description of the Prior Art
In general, projection screens are classified into a front projection screen (reflection type) and a rear projection screen (transmission type). The rear projection screen is very useful because it enables one to compactly constitute a projection apparatus for simply projecting a film for slide, microfiche, 8 m/m and 16 m/m cinefilms. In addition, the rear projection screen is also useful as a large scale screen for video projectors which draw attention in this information oriented age.
Heretofore, most rear projection screens have been so-called coating type screens which are produced by coating a transparent or semi-transparent support with a pigment, glass powders, synthetic resin spheres, aluminum metal powders and the like dispersed in a vehicle. Other types of rear projection screens having some other advantages, i.e. rear projection screens utilizing light scattering effect caused by crystalline particles formed in a crystalline high polymer material, are also known, for example, as shown in Japanese Utility Model Publication No. 14236/1968 and Japanese Patent Publication No. 19257/1973.
When a commercially available crystalline high polymer material is directly made into a rear projection screen, the resulting rear projection screen can not satisfy the advantageous characteristics for a rear projection screen such as excellent light transmission (bright), excellent light scattering transmission, no glare, and high resolution power. Therefore, the commercially available crystalline high polymer material is once melted, shaped into a film, sheet or plate and subjected to a crystal growing heat treatment to impart the desired characteristics. In other words, high polymer materials are generally considered as amorphous, but it is known that high polymer materials rich in regular and symmetric repeating units show crystalline properties. However, the light scattering property usually available is so poor that it is not sufficient for a rear projection screen. Therefore, it is necessary to subject it to a crystal growing heat treatment for the purpose of enhancing the crystallinity or controlling the arrangement state of crystals to obtain excellent optical characteristics, particularly, high light scattering property.
The crystalline state of crystalline high polymers largely depends on the crystallization condition upon crystallizing from the melted state, i.e. heat hysteresis, as well as easiness of crystallization of the molecule itself constituting the polymer, to a great extent. For example, when the crystalline high polymer is gradually cooled from the melted state, the crystal size is very large, but when the crystalline high polymer is rapidly cooled, the resulting crystal size is small and transparent and does not show any light scattering property. As mentioned above, the heat hysteresis between melting and crystalline solidification is an important process for determining the crystallinity of a crystalline high polymer, and it is possible to obtain an optional crystallinity, i.e. rear projection screen characteristics, by a heat treatment capable of controlling said process. This heat treating condition contains the treating temperature and the time change. Therefore, a very complicated and accurate heating control is necessary for producing rear projection screens having desired properties with a uniform quality in a mass production, and further such complicated and accurate heating control is disadvantageous from an economical point of view. More particularly, it is very difficult to impart desirable screen characteristics uniformly to the whole surface of the screen upon producing a large rear projection screen for video projectors.
Furthermore, a rear projection screen can be used in various application fields, but the required screen characteristics depend on the purposes and circumstances of use. For example, when it is used for a looking and listening education machine for a group, an excellent light scattering property is required for the purpose of giving a wide field angle and a bright image. On the contrary, when the rear projection screen is used for a studying machine for an individual or a microreader, light is required rather than light scattering property. Therefore, the crystal growing heat treatment conditions vary depending upon each individual required characteristics, and complicated heat treatment conditions should be set and design of the heat treating devices should be changed.
The light scattering caused by crystalline high polymers is reviewed in detail below. The behavior largely depends on the distribution state of the difference of optical characteristics, particularly, refractive index, between the amorphous substance constituting the crystalline high polymer and the crystal portions grown dispersingly therein. In addition, polymers contain not only clear two phases, that is, amorphous state .revreaction. crystal state, but also many intermediate states depending upon the coagulating state of the molecular chains existing nonuniformly in a fine state (from a microscopic point of view). Consequently, when the above mentioned intermediate states are present as intermediate phases, there are differences of refractive indexes such as [amorphous phase-intermediate phase], and [intermediate phase-crystalline phase] smaller than the difference [amorphous phase-crystalline phase]. Therefore, there exist very close differences. In other words, the relation may be shown as follows: [refractive index of amorphous phase &lt; refractive index of intermediate phase M.sub.1 &lt; . . . &lt; refractive index of intermediate phase Mn &lt; refractive index of crystalline phase].
According to prior art methods, the ratio of the content of the intermediate phase high polymer, having an intermediate refractive index between the refractive index of the amorphous phase and that of the crystalline phase, is controlled by a crystal growing heat treatment to give a rear projection screen having desirable optical characteristics. However, the prior art method is generally carried out in a crystallization process from the melted state of the crystalline high polymer material by adjusting the crystal growing temperature and time to control the ratio of intermediate phases formed in the amorphous phase (heat treatment method). Therefore, it is extremely difficult to obtain a large rear projection screen having predetermined desirable optical characteristics, uniform quality, and suitable for mass production.
Conventional rear projection screens utilizing crystalline high polymers have been subjected to the above mentioned heat treatment to adjust the screen characteristics. Consequently, high techniques and precision apparatus are required for obtaining a rear projection screen having satisfactory lightness, light transmission scattering property, resolution power, and glare. Therefore, the conventional rear projection screens are not suitable for mass production and the cost is so expensive that it is commercially of low value.
There is another method for controlling optical characteristics of crystalline high polymer materials by incorporating an additive. For example, Japanese Patent Publication No. 19257/1973 discloses incorporating a finely divided (5-50 microns) crystal deforming agent such as starch, sucrose octoacetate, cellulose acetate butyrate and the like into the crystalline high polymer. The crystal deforming agent is an agent capable of existing without being melted when a crystalline high polymer material is melted and crystallized and deforms a part of the growing direction of the crystal particles so as to increase the light scattering property. The crystal deforming agent can disturb the degree of freedom of crystal growth to deform the shape of the crystals resulting in an improvement in light scattering property.
However, according to the above mentioned method utilizing a crystal deforming agent, the crystal deforming agent does not melt so that light is reflected at the contact interface between the crystalline high polymer and the crystal deforming agent unless their compatibility is excellent and thereby, back scattering is caused which often fails to satisfy the lightness required by the rear projection screen.